“Be patient toward all that is unsolved in your heart and try to love the questions themselves like locked rooms and like books that are written in a very foreign tongue. Do not now seek the answers, which cannot be given you because you would not be able to live them. And the point is, to live everything. Live the questions now.”
-Letters to a Young Poet
“If we wish to be let in on the secrets of life, we must be mindful of two things: first, there is the great melody to which things and scents, feelings and past lives, dawns and dreams contribute in equal measure, and then there are the individual voices that complete and perfect this full chorus.”
~Poet’s Guide to Life (trans. Ulrich Baer)
“You have to live life to the limit, not according to each day but according to its depth. One does not have to do what comes next if one feels a greater affinity with that which happens later, at a remove, even in a remote distance.”
~Poet’s Guide to Life (trans. Ulrich Baer)
“Life goes on, and it goes past a lot of people in a distance, and around those who wait it makes a detour.”
~Poet’s Guide to Life (trans. Ulrich Baer)
“If only we arrange our life according to that principle which counsels us that we must hold to the difficult, then that which now still seems to us the most alien will become what we most trust and find most faithful.”
-Letters to a Young Poet
“There is only a single, urgent task: to attach oneself someplace to nature, to that which is strong, surviving and bright with unreserved readiness, and then to move forward in one’s efforts without any calculation and guile, even when engaged in the most trivial and mundane activities.”
~Poet’s Guide to Life (trans. Ulrich Baer)
“And yet, is this not what life is? This is what I think: that the countless paltry, timid, petty, and shameful details ultimately still amount to wonderful whole- a whole that would not exist if it depended upon us to understand and achieve it, but to which we contribute in equal parts with our abilities and failures.”
Poet’s Guide to Life (trans. Ulrich Baer)
“I am learning to see. I don’t know why it is, but everything penetrates more deeply into me and does not stop at the place where until now it always used to finish. I have an inner self of which I was ignorant. Everything goes thither now, what happens there I do not know.”
-Notebooks of Malte Laurids Brigge
“At bottom no one in life can help anyone else in life; this one experiences over and over in every conflict and every perplexity: that one is alone.”
-Letters to a Young Poet
“And yet life is transformation: all that is good is transformation and all that is bad as well. For this reason he is in the right who encounters everything as something that will not return.”
-Poets Guide to Life (trans Ulrich Baer)
“Oh, how I believe in it, in life. Not that which makes up our time, but that other, the life of little things, the life of animals and of the great plains.”
-Selected Letters of Rainer Maria Rilke
“Life is very singularly made to suprise us (where it does not utterly appall us).”
-Selected Letters of Rainer Maria Rilke
“And we: spectators, always, everywhere, looking at everything, and never from!…Who’s turned us around like this, so that whatever we do, we always have the look of someone going away? Just as a man on the last hill showing him his whole valley one last time, turns, and stops, and lingers -so we live, and are forever leaving.
-The Duino Elegies: The Eighth Elegy
“For believe me, the more one is, the richer is all that one experiences.”
-Letters to a Young Poet
“Explore transformation throughout. What is your most suffering experience? Is drinking bitter to you, turn to wine.”
-Selected Letters of Rainer Maria Rilke
“I am learning it daily, learning it through pains to which I am grateful: patience is all!”
-Letters to a Young Poet
“I have patience for centuries in me and will live as though my time were very big.”
-Selected Letters of Rainer Maria Rilke
“Nothing in the world can one imagine beforehand, not the least thing. Everything is made up of so many unique particulars that cannot be foreseen.”
-Notebooks of Malte Laurids Brigge
“Resolve to be always beginning -to be a beginner!”
-Letters to a Young Poet
“Children rest in love…but then they are still pure in their illusion that it is possible to belong to someone; and when they say ‘mine’, they make no claim to ownership; they hug and let go, and when they do hold on, it is God with whom they are still enmeshed, who draws the others to himself through there innocently open arms.”
-Rilke and Benvenuta: an Intimate Correspondence
“I live not in dreams but in contemplation of a reality that is perhaps the future.”
-Selected Letters of Rainer Maria Rilke
“I feel that to work is to live without dying. I am full of gratitude and joy. For since my earliest youth I have wanted nothing but that.”
-Selected Letters of Rainer Maria Rilke
“Then take your fate upon yourself and bear it, its burden and its greatness, without ever asking for that reward which might come from without.”
-Letters to a Young Poet
“Fate loves to invent patterns and designs. Its difficulty lies in complexity. But life itself is difficult because of its simplicity. It has only a few things of grandeur not fit for us.”
-Notebooks of Malte Laurids Brigge
“Never think destiny’s more than the substrate of childhood: how often you’d catch up with a lover, panting, panting, from the happy chase, into the open forever.”
-The Duino Elegies: The Seventh Elegy
The Rainer Maria Rilke Archive 
Where exactly is this found in the Letters: “Resolve to be always beginning -to be a beginner!”
That is a great question! This entire time I thought it came from Letters to a Young Poet, specifically Letter Four, but after some digging it actually comes from Letters to Merline. Thanks!
Here is the actual passage:” If the Angel deigns to appear, it will be because you have convinced him, not by tears but by your humble resolve to be always beginning — to be a Beginner!”